In Ranked Roles, each player picks a role—support, mid lane, safe lane, etc—before queuing for a match. The matchmaker will then put five players together so that each team has one safe lane, one mid lane, one off lane, and two support players.

It ensures more balanced, structured games in which you can rely on your teammates to do their job. If you stray from your role, you can be reported by your team—but I reckon most players using it want to take it seriously. Valve didn't say in its blog post whether Ranked Roles is a permanent or temporary addition to Dota Plus.
____________________Source: pcgamer